The beauty and complexity of living matter begins with the first steps of growth and with the formation of cell clusters and tissues. At the biological level, the processes are complex and often unknown, not measured and quantified, except perhaps in botanics. However quite general ideas can be given using a pragmatic approach based on symmetries following a "Landau" approach. Using the elasticity of soft tissues, eventually taking into account the existence of fibres, shapes can be explained by a variational treatment where incompressibility of tissues is treated via the existence of a stream function as in hydrodynamics. Despite the complexity of the formalism, especially in 3D, such an approach allows sometimes to answer open questions, in the biology of development. As examples I will describe grass blades, green algae, the sympetalous flowers and olive trees.

This is a joint work with Julien Dervaux and Martin Mueller.